vendor: Bundle golang.org/x/net/context

Generated with:

  $ make install.tools
  $ make update-deps
  $ git checkout HEAD -- vendor/github.com/spf13/pflag
  $ git checkout HEAD -- vendor/github.com/runtime-spec

I rolled back the other changes because I haven't checked for
compatibility issues due to the upgrades.  I also rolled back the hash
bumps for those packages in glide.lock.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking